Fetuses of both genders start with androgynous genitalia that develops over time. The only way to determine sex during a sonogram is by visualizing developed genitals, according to Fahimeh Sasan, Professor of Obstetrics, Gynecology and Reproductive Science at Icahn School of Medicine. Correctly predicting the sex of the baby depends on which type of ultrasound you choose, the position of the baby inside the uterus and the stage of your child's genital development.
All fetuses begin with the same external genitals, exhibiting what looks like an undeveloped vagina called a urogenital fold with an over-sized clitoris called a genital bud. And while your baby's head develops first, their genitals won't change until the ninth week of development, when sex-determining hormones shape the androgynous genitalia into either a penis or vagina, enlarging the genital bud into the glans penis of the male or shrinking it to form the clitoris of the female. The urogenital fold either fuses to make the scrotum or becomes more slender to form the labia. Genital development continues throughout pregnancy, but with the right ultrasound, you can see your baby's sex by the 11th week with some degree of confidence.
It's easier to identify a male by protruding external genitalia. The ultrasonographer looks for the genital bud as it extends up and away from a boy's body. The urogenital fold descends and forms a discernible scrotum, but the testes do not appear inside the scrotum until the third trimester. Until then, the ultrasonographer must identify a hanging sack. If there's any doubt, a light tap to the mother's belly can induce a sway in the loose skin for identification. In some cases, a male can be identified by a pee stream traveling through the newly formed penis. The labia form three noticeable streaks between the legs of a female baby that are most easily seen on the baby's rear with her legs tucked up into her body. The shrinking genital bud also points down and inward, signifying a girl.
A transducer, the wand-like instrument that the ultrasonographer places on the pregnant belly, sends ultra high-pitch sound waves into the uterus to form a two-dimensional image. The technician rubs a clear jelly on the mother's belly to help the wand move and to aid in the image-forming sound's penetration. You may ask the ultrasonographer to predict the baby's gender and the technician will look for the telltale signs, but a two-dimensional ultrasound is slightly limited because the baby must be in a proper position. The baby's legs and even the umbilical cord can obstruct the view of the genitals. However, even with a clear view, identifying the baby's sex in the first trimester comes with only a 53 percent success rate, according to the textbook, Ultrasound in Obstetrics and Gynecology, Volume 1. At 16 weeks, when the genitals have had time to form, that success climbs above 90 percent.
Three-dimensional ultrasounds are much better at predicting the sex of the baby and they are capable of prediction much earlier, exhibiting an 85 percent success rate in the first trimester at 11 weeks. The 3-D images are created by compiling the data of many two-dimensional ultrasound images into one three-dimensional model. This is able to overcome difficulties with the baby's positioning to make a clear model of the genitals. A three-dimensional ultrasound may require a specially designed transducer that is inserted into the mother's vagina while a separate transducer is used simultaneously on her belly.